Scientists are revising the history of one of the world’s most important crops. Drawing on genetic and archaeological evidence, researchers have found that a predecessor of today’s corn plants still bearing many features of its wild ancestor was likely brought to South America from Mexico more than 6,500 years ago. Farmers in Mexico and the southwestern Amazon continued to improve the crop over thousands of years until it was fully domesticated in each region.

She’s quick to add that there is no direct evidence of ancient Maya pirates, but given how much valuable cargo was passing through these ports—and the fact that boats were often depicted carrying armed warriors—it’s a pretty good guess. Chances are, piracy started long before Europeans arrived. The presence of pirates of the ancient Caribbean might also explain the tall pyramid that could serve dual functions: for religious ceremonies and as a lookout. Vista Alegre had another quirk. In pre-Columbian times, the settlement was on a peninsula, with three sides facing the water (the bays on either side have since filled in) and an elaborate stone wall, probably topped by a wooden barrier, guarding the south-facing fourth side. But there appears to be no roads or settlements in that direction. Why build a wall if there’s no one on the other side to keep out?

The wall was likely meant to keep out marauders who, worried about being spotted on the water, would land nearby and try to enter the city by land. Many of the sites along the coast have similar walls to Vista Alegre. The ruins of Isla Cerritos, 100 kilometers to the west, had a 305-meter wall that blocked the harbor like a giant arm shielding the city.

Beyond physical defenses, little else is known about coastal battles of the Maya world. Were there trained ocean fighters? Specially designed warships? Naval strategies? In fact, of the thousands of boats that once littered the coasts, all that has survived is a single canoe preserved in peat soil that dissolved almost as soon as it touched the air and a couple of paddles discovered in Belize. Yet, according to the first Spaniards to visit the coast, the Maya used many types of boats with up to 25 paddlers and capable of hauling over three tonnes of material, which is more than some midsize pickups can handle today.

Some canoes may have even had sails. Several Spanish chroniclers describe seeing sails along the coast—there are no images, however, of sails among the codices and murals of the Maya themselves. Many local people still build traditional boats and even host international paddle races, but it’s tough to know what parts of these traditions are old and which are new.

Modern Maya canoe builders have specific rules about construction. “For example, when to cut down the tree, which tree,” says Mariana Favila, an anthropologist at the National Autonomous University of Mexico, who is writing her PhD thesis on the maritime practices of the Maya, partly by spending time with modern indigenous communities.

She points out that there are many traditions around boatbuilding that seem to predate Europeans, such as a ban on women being near a boat while it’s being built. Local tradition holds that women are associated with cold, whereas men are characterized by heat. Thus, for boats—which are also tied to cold—to stay afloat, they must be kept separate from women.

When most people imagine an ancient Maya city, they probably picture Chichén-Itzá with the commanding El Castillo at its center, surrounded by ball courts of varying sizes and wide plazas. But the sites we see today were the ceremonial centers of the cities, not where people lived. Studying a pyramid is like trying to understand the United States by digging up the Lincoln Memorial. It tells you something about values and traditions, but not daily life.

For that, you need to see where the ancient Maya lived and worked. Modern satellite images and laser-guided surveys from planes have revealed that Maya cities were far more sprawling than anyone would have ever guessed. Earlier this year, a large multinational team announced that several cities in northern Guatemala once presumed separate were, in fact, connected by 60,000 previously unseen structures.

Many of these were wooden houses built on low platforms. But, like giant, ravenous beasts, the tropical landscapes of the Yucatan and Petén Basin gobbled up all the wooden structures they touched, along with tools and boats. Except in one place—an ancient lagoon in Payne’s Creek National Park, a nature preserve encompassing savannah, tropical rainforest, and mangroves in southern Belize.

Heather McKillop, an archaeologist at Louisiana State University, first came to Payne’s Creek in 2004 on an odd sort of archaeological scouting expedition. She knew that there had been settlements in the area and that the ocean had risen since then. So rather than shovels and pickaxes, she brought a swimsuit, a couple of buckets for specimens, and a vague notion that the rich lagoon soils were hiding interesting artifacts probably related to salt making. She and a few students waded into the water and quickly found all sorts of ceramic shards from pots used to boil briny seawater to collect salt and the occasional wooden stump—probably either a dead tree or driftwood washed in from storms.

“Wood doesn’t preserve in the rainforest,” McKillop says. “I thought, oh they’re just old tree roots or they’re pieces of wood that have drifted in.” There’s no way they were left over from the Maya, that would be ridiculous. Termites, shipworms, or thousands of other creatures that scour the jungles and shores for detritus quickly devour wood here. But just out of curiosity, she and her students decided to dig up one.

“I said, if it’s a post, it’s going to go straight down and the end is going to be sharpened. If it’s a tree, it’s going to branch out,” she says, assuming at the time they were all dead trees. “I just want to settle this once and for all.”

They took turns holding their breath, ducking a meter underwater, and scraping around the sides of the logs. The soil was a heavy sort of peat, covered in moss that was perfect for clutching while digging with the other hand. Finally, they got to the bottom and freed the wood, pulling it up to the surface.

“The four of us are looking at it, and the end comes out of the water. And it’s sharpened,” she says. It was like reaching into a wastebasket in Rome and pulling out a poem by Julius Caesar. No one had ever found something like this. “Then they all looked at me and say, ‘So what is it?’”

That same trip, they found more than a dozen sites along the creek and the first-ever intact Maya canoe paddle. Since then, McKillop and her team have found more than 4,000 such posts—perfectly preserved in the oxygen-deprived peat—in a five-square-kilometer area, using masks and snorkels while floating facedown on cleverly modified pool loungers.

The sheer number of posts in such a small area suggests that the coastline once bustled with salt works. Salt is crucial to all civilizations, not just as food but also as a preservative. Experts always assumed the Maya collected it along the shoreline and passed it inland, but now they had evidence of whole communities dedicated to producing salt.

“If you sat on the beach, you would watch two boats go by every single day full of salt. It’s not an occasional thing, this would be a very regular transport schedule,” says Thomas Guderjan, an archaeologist at the University of Texas at Tyler who specializes in Maya agriculture and city planning. “You could not service that population just with an occasional guy who takes a paddle up to get salt. This would be a very structured market.”

No one knows for sure how the ancient Maya economy worked, but trade was likely crucial. Quetzal feathers (found in tropical cloud forests) came downhill, jade (found in the south) went north, shells went inland to the mountains of central Mexico, and obsidian (found in the volcanic spine of the continent) went to the flatlands of the Yucatan. There were no pack animals or wheels for carts, and boats packing a few tonnes of material would have been highly efficient.

But not everyone is so convinced by this picture. Rafael Cobos Palma is an archaeologist at the Autonomous University of the Yucatan with perhaps more experience investigating the coastal Maya than anyone else alive. He doesn’t see some kind of market-based trade but something more like a pipeline completely controlled by elites from Chichén-Itzá.

“Trade is not the correct word to use,” Cobos Palma says. “Nobody else participated. It was a closed system.”

He compares the gold and turquoise coming along the coast to Fabergé eggs created for Russian royalty during the turn of the last century. It’s not really trade if the materials go only in one direction. He admits that there might have been other, less-formal trade systems that siphoned goods to a few other cities, but insists this isn’t because of a market, it’s from leaks in the pipeline.

It’s a controversial idea, considering how long the ports operated and the variety of goods the coastal Maya moved and how different their culture seems from their inland neighbors. But it gets at a fundamental question about the ancient Maya. Did they have an economy the way we think of it—with currency, merchants, and markets? Or was it controlled by the iron fist of the state? Perhaps the answer lies in one of the many unexcavated sites along the coast, like Xel-Ha or Xcalak.

There’s just one problem: many of these sites are underneath multimillion-dollar resorts.

Ironically, the global fascination with the ancient Maya that puts pyramids on magazine covers and in Indiana Jones movies has helped fuel a tourism industry gobbling up wide swaths of the coastline, including many Maya sites. Xel-Ha, an ancient city north of Tulum, is now an amusement park, with playgrounds, rides, and concession stands. Others are in the middle of golf courses, and one, a rare cave shrine, is poking up through a Home Depot parking lot. When Rissolo wanted to see an important shrine at Xcaret, near Playa del Carmen, he had to buy tickets and walk past the souvenir shops and a dolphin tank to get there.

Thankfully, many sites were visited and cataloged by archeologists before the all-inclusive resorts moved in, though most of these studies were just cursory. Still, someday archaeologists may be able to finish what they started.

“The bulldozer might have destroyed one or two platforms here or there but there’s certainly more information. And fortunately, all that information is underneath,” Cobos Palma says.

More destructive might be time itself. Cobos Palma points out that the coast of Tabasco, just west of Vista Alegre, is suspiciously short on ancient sites. The best explanation is that they have been buried under soil or rising seas. One of his research sites, Uaymil, was once on the coast but is now more than a kilometer inland. McKillop’s research area was originally on solid ground until the oceans rose up around it. Others could have been wiped out by a single well-placed hurricane.

But there’s still too much to find to despair about what’s been lost. Some of the most important coastal ports may remain undiscovered.

Back at Vista Alegre, far from the lively coastline of Cancún, Rissolo and Glover finish their survey, and we pile back into our skiff. Unlike the east coast of the Yucatan Peninsula, the northern end is nearly deserted. It’s a beautiful day, with enough wind to kick up some sea spray but it’s still pleasant. The previous week, the area had been buffeted by powerful winds as Subtropical Storm Alberto formed offshore and charged toward Florida. Roads in the area have been flooded for days and Cancún ground to a standstill. And that was just a small storm. Imagine a powerful monster like Hurricane Wilma—which devastated the region in 2005—and it’s easy to see why not everyone has embraced the coast.

“This entire coastline is unpopulated,” says Rissolo, looking west toward the Gulf of Mexico. “From an archaeologist’s perspective, there’s really a great opportunity to study how the ancient Maya organized themselves on the landscape. Imagine trying to do what we’re doing when you’ve got golf courses, all-inclusive resorts, and shopping malls that run all the way from Cancún to Tulum.”

On the way back to the harbor, they decide to take a little detour. Locals say there is perhaps evidence of a settlement on the nearby island of Holbox, an emerging vacation spot for off-the-beaten-path tourists. One archaeologist documented the site in the 1950s, but no other scientist has visited the place. Rumor has it there are a few stone structures and maybe a shrine. Somewhere. Glover pulls out a drone and attempts to launch it.

“That’s a stiff wind, dude,” he says to no one in particular, testing the drone’s rotors. “Not ideal.”

“We don’t even know if it’s in the interior. Is it at the edge of the mangroves? The coastline is just so complex,” Rissolo mutters.

Glover eventually gets the drone airborne and battles the wind for a while, taking video he hopes will later reveal the site. It’s bizarre that there could be an undocumented ancient settlement 100 meters from our boat and we can’t see it. But then again, that’s the story of modern Maya archaeology. In recent years, the biggest discoveries haven’t been pyramids or kings but pedestrian things: ancient farm fields, roads, and salt factories. Seeing how the Maya constructed their civilization, in addition to their sacred buildings, has broadened our view of how their world looked. It was a world of mysterious ceremonies and unusual head shapes, but also one of people who were adept at getting what they wanted quickly and efficiently.

Eventually the researchers give up—Glover is wary of running out of battery power and crashing the drone into the water. They agree to come back, perhaps with kayaks, and inspect the mangroves more carefully.

They fire up the engine and motor toward the mainland. The clouds are almost gone, and tomorrow promises to be another perfect day in paradise. Boatmen will cart tourists to the island, just as they have for decades, while the fishermen load up their nets to leave at first light—just as they have for thousands of years.

Part 1

For thousands of years, ancient Maya kings ruled a vast inland empire in Mexico and Belize. But just how inland was it, really?

Vista Alegre, a ruin of a town near the northern tip of the Yucatan Peninsula in Mexico, was once a bustling outpost. Dozens of canoes crowded the harbor, loaded down with dyes from the west, jade from the south, and obsidian from mountains hundreds of kilometers away. The sound of trumpeting conch shells periodically sliced the air—an alert from sentries scanning the horizon from platforms attached to stone structures. The call signaled an incoming boat—to trade or, perhaps, to plunder.

Within the town, the smell of fish hung heavy in the air as fishermen hurried about with their catches slung across their backs. They passed a man outside his hut hacking a pile of decorative shells into portable sizes for the next outgoing canoe. In another hut, a woman was using salt from a town to the south to dry freshly caught fish that would then be shipped to cities far away. And all the while, smoke from a signal fire atop a pyramid guided exhausted ocean travelers to safe harbor.

Today, a thousand years later, the town isn’t much to look at. Centuries of accumulated dirt and vegetation cover the pyramids. Trees growing on various structures have succumbed to gravity, tumbling and taking with them massive stone blocks once perfectly fitted together.

Today, the ruins of Vista Alegre, on the northern Yucatan coast, are barely visible but still reveal much about the ancient maritime Maya. Video courtesy of Jeffrey Glover and Dominique Rissolo

Though part of the ancient Maya world, which stretched from here all the way into modern-day El Salvador, Vista Alegre lacks the grandeur of many sites. It doesn’t have the dozens of glistening pyramids that lure millions of tourists to Chichén-Itzá, the enticing carvings of Palenque, or the vibrant murals of Bonampak. In fact, were it not for a single pyramid in the middle of a handful of crumbling structures, you might miss its human past altogether. But this small port town at the bottom of the Gulf of Mexico was once part of a complex network of coastal commerce that dominated the ancient world but has been understudied by academics and ignored by the public. Because, unlike Chichén-Itzá or Palenque or Bonampak in the interior of the country, time has nearly erased coastal ports like Vista Alegre from history.

It’s part of a growing collection of archaeological sites revealing a complex and cosmopolitan network of sea traders with their own culture and traditions, who are at once separated from and deeply tied to their more famous compatriots deep in the Yucatan jungles.

“Our knowledge of markets and the role of markets is changing in the Maya area,” says Jeffrey Glover, an archaeologist at Georgia State University. Glover stands on a cluster of exposed blocks at the top of Vista Alegre’s steep central pyramid with its commanding view of the coastline. Looking out, he seems to see the town as it once was, even describing it in the present tense. “There are a lot of people there that need stuff and that want stuff. And they’re probably getting gold from as far south as Panama and Costa Rica, with turquoise that’s coming from the American Southwest.”

Glover and his colleague Dominique Rissolo have been exploring this small site for more than a decade. When they first came to this undeveloped coastline over two hours northwest from Cancún by car and then boat, they expected a small town dependent on the fortunes of the much larger Chichén-Itzá—a nameless cog in the machinery of a great city only 125 kilometers away. But that’s not what they found. Vista Alegre predates the larger city by hundreds of years. The people here ate differently, had different fashions, and traded an astounding diversity of precious things from around Meso-America.

And there was something else here that the archeologists did not expect. The town’s central 10.6-meter-high pyramid is oddly steeper than others in the region—made possible by an unusual concrete recipe that connects its stones and is usually found hundreds of kilometers to the west. This innovation, presumably gleaned from passing travelers, allowed them to build a pyramid tall enough to let them see for a great distance in all directions but with fewer blocks than typical pyramids.

“It blew me away when I saw it. I hadn’t seen anything like that anywhere in this region,” says Rissolo, a researcher at the University of California, San Diego. “There’s a certain kind of cosmopolitan nature to the site, where people are exposed to different styles and different traditions.”

As we walk the grounds, Rissolo regularly leans down to pick up bits of pottery, some 1,500 years old. It turns out that Vista Alegre was just one of dozens of small settlements that rim the Yucatan Peninsula at about 40-kilometer intervals, the distance a team of paddlers can move a goods-laden boat in a day. None of the coastal communities had the grandeur of the bigger Maya cities, but when put together, they paint a vivid picture of commerce in the region. And, in another surprise, of combat.

The Maya are often called “people of corn” because their identity is forever tied to that crop. Academic and popular stories paint pre-Colombian Maya as a peaceful, inland people who tended crops, feared the sea, and looked to the stars. As with all historical generalizations, however, this picture has faded with new discoveries. Modern scientists have found that the real ancient Maya were just as war-prone as Aztecs or Europeans and perfectly comfortable on the sea.

Looking at the bones from sites like Vista Alegre, scientists now know the ancient coastal Maya ate healthier than their inland compatriots, getting more protein from sea life. Evidence suggests they were more egalitarian too, though less sophisticated in their cultural traditions. Take the common ancient practice of head shaping (whereby boards are tied to an infant’s head to cause it to grow a certain way). Whereas people in Calakmul, near the border of modern-day Guatemala, had a myriad of head shapes fashioned from birth, those on the coast kept it simple—just a slightly elongated head. And rather than elaborate jade- or obsidian-inlaid teeth, coastal people tended to simply sharpen them down a little. But that’s not to say they were boring.

The people came from all over, says Vera Tiesler at the Autonomous University of Yucatan. “There was a big melting pot of people there, traveling down the coast.”

Tiesler is a bioarchaeologist. Rather than digging up pyramids or looking for jade jewelry, she studies human remains. From diseases to wounds to DNA, a set of bones can tell you more about people than a stone structure. And while the Maya coastline may not have a lot of fancy pyramids or ornate paintings, it has a lot of bones littered around—the soil composition and high salt levels lead to wonderful bone preservation.

When a Mayanist finds a set of bones, it often finds its way to Tiesler. And after looking at hundreds of ancient skeletons, she’s come to see the coastal Maya as a sort of subculture. Their seafood diet—revealed through bone analysis—suggests that over generations, families spread long distances along the coast but rarely went inland. It was as if once they found the coast, they didn’t want to leave it.

Coastal Maya also fought differently than their inland compatriots. Contrary to their reputation, the ancient Maya engaged in plenty of warfare. Armies would face off with clubs, hatchet-like weapons, and throwing spears called atlatls. In hand-to-hand combat, injuries were mostly sustained on warriors’ left sides from right-handed assailants. But Tiesler says battle scars of the ancient coastal Maya favor no side. Also, ancient armies were usually all male, but a higher-than-expected number of the violent casualties Tiesler sees were women. Plus, many combatants survived, which would be odd on a battlefield. The likely reason for these discrepancies between Maya armed conflict wounds and these coastal casualties? Piracy.

Salt is essential for life. As ancient civilizations evolved from hunters and gatherers to agrarian societies, it has not been clear how people acquired this mineral that is a biological necessity. However, an anthropologist at LSU discovered remnants of an ancient salt works in Belize that provide clues on how the ancient Maya at the peak of their civilization more than 1,000 years ago produced, stored and traded this valuable mineral.

New technology allows scientists to visualize ancient Maya cities like never before

In the autumn of 1929, Anne Morrow Lindbergh and her husband Charles flew across the Yucatán Peninsula. With Charles at the controls, Anne snapped photographs of the jungles just below. She wrote in her journal of Maya structures obscured by large humps of vegetation. A bright stone wall peeked through the leaves, “unspeakably alone and majestic and desolate — the mark of a great civilization gone.”

Nearly a century later, surveyors once again took flight over the ancient Maya empire, and mapped the Guatemala forests with lasers. The 2016 survey, whose first results were published this week in the journal Science, comprises a dozen plots covering 830 square miles, an area larger than the island of Maui. It is the largest such survey of the Maya region, ever.

The study authors describe the results as a revelation. “It’s like putting glasses on when your eyesight is blurry,” said study author Mary Jane Acuña, director of El Tintal Archaeological Project in Guatemala.

In the past, archaeologists had argued that small, disconnected city-states dotted the Maya lowlands, though that conception is falling out of favor. This study shows that the Maya could extensively “exploit and manipulate” their environment and geography, Acuña said. Maya agriculture sustained large populations, who in turn forged relationships across the region.

Combing through the scans, Acuña and her colleagues, an international 18-strong scientific team, tallied 61,480 structures. These included: 60 miles of causeways, roads and canals that connected cities; large maize farms; houses large and small; and, surprisingly, defensive fortifications that suggest the Maya came under attack from the west of Central America.

“We were all humbled,” said Tulane University anthropologist Marcello Canuto, the study’s lead author. “All of us saw things we had walked over and we realized, oh wow, we totally missed that.”

Preliminary images from the survey went public in February, to the delight of archaeologists like Sarah Parcak. Parcak, who was not involved with the research, wrote on Twitter, “Hey all: you realize that researchers just used lasers to find *60,000* new sites in Guatemala?!? This is HOLY [expletive] territory.

Parcak, whose space archaeology program GlobalXplorer.org has been described as the love child of Google Earth and Indiana Jones, is a champion of using satellite data to remotely observe sites in Egypt and elsewhere. “The scale of information that we’re able to collect now is unprecedented,” Parcak said, adding that this survey is “going to upend long-held theories about ancient Maya society.”

With support from a Guatemala-based heritage foundation called Pacunam, the researchers conducted the massive and expensive survey using lidar, or light detection and ranging. They mapped several active archaeological sites, plus well-studied Maya cities like Tikal and Uaxactun.

Lidar’s principles are similar to radar, except instead of radio waves lidar relies on laser light. From an aircraft flying just a few thousand feet above the canopy, the surveyors prickled each square meter with 15 laser pulses. Those pulses penetrate vegetation but bounce back from hard stone surfaces. Using lidar, you can’t see the forest through the invisible trees.

Beneath the thick jungle, ruins appeared. Lots and lots of them. Extrapolated over the 36,700 square miles, which encompasses the total Maya lowland region, the authors estimate the Maya built as many as 2.7 million structures. These would have supported 7 million to 11 million people during the Classic Period of Maya civilization, around the years 650 to 800, in line with other Maya population estimates.

“We’ve been working in this area for over a century,” Canuto said. “It’s not terra incognita, but we didn’t have a good appreciation for what was really there.”

Archaeologist Arlen Chase, a Maya specialist at the University of Nevada at Las Vegas who was not involved with this survey, said for years he has argued that the Maya society was more complex than widely accepted. In 1998, he and archaeologist Diane Chase, his wife, described elaborate agricultural terraces at the Maya city of Caracol in Belize. “Everybody would not believe we had terraces!” he said.

He gets much less push back now, he said. “The paradigm shift that we’ve predicted was happening is in fact happening” Chase said, which he credits to lidar data. He has seen lidar evolve from a “hush-hush type of technology” used by the military to map Fallujah streets to a powerful archaeological tool.

Chase, who previously used lidar at Caracol, where as many as 100,000 people lived, compares this technology to carbon-14 dating. Radiocarbon dating gives archaeologists a much more accurate timeline. Lidar is about to do the same for archaeologists’ sense of space, particularly in densely forested areas near the equator. Two years ago, researchers used lidar mapped dense urban infrastructure around Angkor, the seat of the medieval Khmer Empire in Cambodia.

“We’re just getting started in so many major sites around the world, whether it’s Angkor Wat, whether it’s Tikal in Central America or major sites in Egypt,” Parcak said.

For all its power, lidar cannot supplant old-fashioned archaeology. For 8 percent of the survey area, the archaeologists confirmed the lidar data with boots-on-the-ground visits. This “ground truthing” suggests that the lidar analysis was conservative — they found the predicted structures, and then some.

“There is still much more ground to cover and work to do,” said Acuña, who will continue to study the large ancient Maya city of El Tindal.

Could you imagine, Canuto said, what might be found through a lidar survey of the Amazon? With technology like this, no forested frontiers are final.

Your Hershey bar may have been worth its weight in gold in Mayan times. A new study reveals that chocolate became its own form of money at the height of Mayan opulence—and that the loss of this delicacy may have played a role in the downfall of the famed civilization.

The study is on the right track, says David Freidel, an anthropologist and Maya expert at Washington University in St. Louis, Missouri, who was not involved with the work. Chocolate “is a very prestigious food,” he says, “and it [was] almost certainly a currency.”

The ancient Maya never used coins as money. Instead, like many early civilizations, they were thought to mostly barter, trading items such as tobacco, maize, and clothing. Spanish colonial accounts from the 16th century indicate that the Europeans even used cacao beans—the basis for chocolate—to pay workers, but it was unclear whether the substance was a prominent currency before their arrival.

To find out, Joanne Baron, an archaeologist with the Bard Early College Network—a network of schools that focus on college-level teaching for high school–aged students—in Newark, New Jersey, analyzed Mayan artwork. She focused on published research and other available Maya images during the Classic Maya period from about 250 C.E. to about 900 C.E. in the southern Maya lowlands in modern-day Mexico and Central America. The objects—including murals, ceramic paintings, and carvings—depict typical market exchanges and tribute payments to Maya kings.

Chocolate didn’t pop up much in the earliest art, Baron found, but it became more prevalent by the 8th century C.E. That’s also around the time people seem to be using it as money—that is, an item widely accepted as payment for goods or services rather than a one-off barter. The Maya usually consumed their cacao as a hot drink, a steamy broth served in a clay cup. One of the earliest depictions of it used in exchange dates to the mid-7th century. In a painted mural displayed in a pyramid that may have been a central marketplace near the Guatemalan border, a woman offers a bowl of what looks like frothing hot chocolate to a man in return for dough used for making tamales. This early depiction suggests that although chocolate was being bartered at this point, it may not have been traded as a form of currency, Baron says.

But later evidence shows that chocolate became a little more like coins—in the form of fermented and dried cacao beans. Baron documented about 180 different scenes on ceramics and murals from about 691 C.E. through 900 C.E. which show commodities delivered to Maya leaders as a tribute, or a kind of tax. Goods like tobacco and maize grain are sometimes given as tribute, but the items that pop up most in these scenes are pieces of woven cloth and bags labeled with the quantity of dried cacao beans they contain, she reports in Economic Anthropology.

Baron believes the fact that Maya kings collected cacao and woven cloth as tax shows that both had become a currency at this point. “They are collecting way more cacao than the palace actually consumes,” she says, adding that the surplus was probably used to pay palace workers or to buy things at the marketplace.

Freidel says cacao was almost universally loved by the Maya. But it would have been far more prized than crops like maize because cacao trees are susceptible to crop failure and didn’t grow well near Maya cities.

Some scholars believe drought led to the downfall of the Classic Maya civilization. Baron speculates that the disruption of the cacao supply which fueled political power may have led to an economic breakdown in some cases.

Freidel says the rise in artistic depictions of cacao may not necessarily indicate increased importance as a currency. As the Classic Maya period unfolded, he says, more and more people wrote things down and painted murals or pottery scenes. “Is it actually getting more important or are we just learning more about it?”

He is also skeptical that the loss of cacao contributed to the Maya’s downfall. Cacao beans were not the only type of currency, Freidel notes—woven cloth and other goods like maize grain or certain types of green stone were also possibly used as money. “My guess is that one commodity crashing would not cause the system to crash.”

Of all the plants that humanity has turned into crops, none is more puzzling than the sweet potato. Indigenous people of Central and South America grew it on farms for generations, and Europeans discovered it when Christopher Columbus arrived in the Caribbean.

In the 18th century, however, Captain Cook stumbled across sweet potatoes again — over 4,000 miles away, on remote Polynesian islands. European explorers later found them elsewhere in the Pacific, from Hawaii to New Guinea.

The distribution of the plant baffled scientists. How could sweet potatoes arise from a wild ancestor and then wind up scattered across such a wide range? Was it possible that unknown explorers carried it from South America to countless Pacific islands?

An extensive analysis of sweet potato DNA, published on Thursday in Current Biology, comes to a controversial conclusion: Humans had nothing to do with it. The bulky sweet potato spread across the globe long before humans could have played a part — it’s a natural traveler.

Some agricultural experts are skeptical. “This paper does not settle the matter,” said Logan J. Kistler, the curator of archaeogenomics and archaeobotany at the Smithsonian Institution.

Alternative explanations remain on the table, because the new study didn’t provide enough evidence for exactly where sweet potatoes were first domesticated and when they arrived in the Pacific. “We still don’t have a smoking gun,” Dr. Kistler said.

The sweet potato, Ipomoea batatas, is one of the most valuable crops in the world, providing more nutrients per farmed acre than any other staple. It has sustained human communities for centuries. (In North America, it often is referred to as a yam; in fact, yams are a different species originating in Africa and Asia.)

Scientists have offered a number of theories to explain the wide distribution of I. batatas. Some scholars proposed that all sweet potatoes originated in the Americas, and that after Columbus’s voyage, they were spread by Europeans to colonies such as the Philippines. Pacific Islanders acquired the crops from there.

As it turned out, though, Pacific Islanders had been growing the crop for generations by the time Europeans showed up. On one Polynesian island, archaeologists have found sweet potato remains dating back over 700 years.

A radically different hypothesis emerged: Pacific Islanders, masters of open-ocean navigation, picked up sweet potatoes by voyaging to the Americas, long before Columbus’s arrival there. The evidence included a suggestive coincidence: In Peru, some indigenous people call the sweet potato cumara. In New Zealand, it’s kumara.

A potential link between South America and the Pacific was the inspiration for Thor Heyerdahl’s famous 1947 voyage aboard the Kon-Tiki. He built a raft, which he then successfully sailed from Peru to the Easter Islands.

Genetic evidence only complicated the picture. Examining the plant’s DNA, some researchers concluded that sweet potatoes arose only once from a wild ancestor, while other studies indicated that it happened at two different points in history.

According to the latter studies, South Americans domesticated sweet potatoes, which were then acquired by Polynesians. Central Americans domesticated a second variety that later was picked up by Europeans.

Hoping to shed light on the mystery, a team of researchers recently undertook a new study — the biggest survey of sweet potato DNA yet. And they came to a very different conclusion.

“We find very clear evidence that sweet potatoes could arrive in the Pacific by natural means,” said Pablo Muñoz-Rodríguez, a botanist at the University of Oxford. He believes the wild plants traveled thousands of miles across the Pacific without any help from humans.

Mr. Muñoz-Rodríguez and his colleagues visited museums and herbariums around the world to take samples of sweet potato varieties and wild relatives. The researchers used powerful DNA-sequencing technology to gather more genetic material from the plants than possible in earlier studies.

Their research pointed to only one wild plant as the ancestor of all sweet potatoes. The closest wild relative is a weedy flower called Ipomoea trifida that grows around the Caribbean. Its pale purple flowers look a lot like those of the sweet potato.

The ancestors of sweet potatoes split from I. trifida at least 800,000 years ago, the scientists calculated. To investigate how they arrived in the Pacific, the team headed to the Natural History Museum in London.

The leaves of sweet potatoes that Captain Cook’s crew collected in Polynesia are stored in the museum’s cabinets. The researchers cut bits of the leaves and extracted DNA from them.

The Polynesian sweet potatoes turned out to be genetically unusual — “very different from anything else,” Mr. Muñoz-Rodríguez said.

The sweet potatoes found in Polynesia split off over 111,000 years ago from all other sweet potatoes the researchers studied. Yet humans arrived in New Guinea about 50,000 years ago, and only reached remote Pacific islands in the past few thousand years.

The age of Pacific sweet potatoes made it unlikely that any humans, Spanish or Pacific Islander, carried the species from the Americas, Mr. Muñoz-Rodríguez said.

Traditionally, researchers have been skeptical that a plant like a sweet potato could travel across thousands of miles of ocean. But in recent years, scientists have turned up signs that many plants have made the voyage, floating on the water or carried in bits by birds.

Even before the sweet potato made the journey, its wild relatives traveled the Pacific, the scientists found. One species, the Hawaiian moonflower, lives only in the dry forests of Hawaii — but its closest relatives all live in Mexico.

The scientists estimate that the Hawaiian moonflower separated from its relatives — and made its journey across the Pacific — over a million years ago.

But Tim P. Denham, an archaeologist at the Australian National University who was not involved in the study, found this scenario hard to swallow.

It would suggest that the wild ancestors of sweet potatoes spread across the Pacific and were then domesticated many times over — yet wound up looking the same every time. “This would seem unlikely,” he said.

Dr. Kistler argued that it was still possible that Pacific Islanders voyaged to South America and returned with the sweet potato.

A thousand years ago, they might have encountered many sweet potato varieties on the continent. When Europeans arrived in the 1500s, they likely wiped out much of the crop’s genetic diversity.

As a result, Dr. Kistler said, the surviving sweet potatoes of the Pacific only seem distantly related to the ones in the Americas. If the scientists had done the same study in 1500, Pacific sweet potatoes would have fit right in with other South American varieties.

Dr. Kistler was optimistic that the sweet potato debate would someday be settled. The world’s herbariums contain a vast number of varieties that have yet to be genetically tested.

“There are more than we could look at in a lifetime,” Dr. Kistler said.

For his part, Mr. Muñoz-Rodríguez plans on searching for more wild sweet potato relatives in Central America, hoping to get more clues to how exactly a thin-rooted weed gave rise to an invaluable crop.

Working out the history of crops like this could do more than satisfy our curiosity about the past. Wild plants hold a lot of genetic variants lost when people domesticated crops.

Researchers may find plants they can hybridize with domesticated sweet potatoes and other crops, endowing them with genes for resistance to diseases, or for withstanding climate change.

Caption1 The distribution of the sweet potato plant has baffled scientists. How could the plant arise from a wild ancestor in the Americas and wind up on islands across the Pacific? Karsten Moran for The New York Times

Caption2 Different varieties of sweet potato on display at the International Potato Center in Lima, Peru. The sweet potato originated in the Americas and spread across the globe. Robert Scotland